The purpose of this research is to determine the cytological effect of the gonadotropin, human chorionic gonadotropin (HCG), on the Sertoli cells of the canine testis. Known concentrations of HCG will be perfused in vivo in the vascularly isolated testis of the anesthetized dog. The testis will be fixed in situ after the administration of HCG by perfusion with formaldehyde-glutaraldehyde-picric acid (FGP). Freeze-fracture and freeze-etch replicas, as well as routinely prepared specimens for studying the Sertoli cell organelles, will be examined by transmission electron microscopy. Lanthanum nitrate, an electron dense tracer, will be used to determine the patency of the Sertoli:Sertoli tight junction. The information from the tracer studies will be correlated with the results obtained by freeze-fracture studies of these junctional complexes in normal and HCG treated, adult and prepuberal dogs. The complex junction between Sertoli cells represents the main component of the blood-testis barrier. The establishment of this barrier coincides with the initiation of spermatogenesis and effectively isolates the later stages of spermatogenesis from the interstitial fluids. How these junctions between Sertoli cells are modified to permit the ascent of the developing germinal elements from the basal tubular compartment toward the lumen is not known. In a preliminary experiment, HCG cytologically altered the Sertoli:Sertoli complex junction. A study of this alteration may result in a better understanding of the role of the Sertoli cell in the control of spermatogenesis. From a medical viewpoint this information could be instrumental in the development of a new approach in the control of fertility and the treatment of sterility.